Nak-Sam Choi

Influence of fibre weight fraction on failure mechanisms of poly(ethylene terephthalate) reinforced by short-glass-fibres

Failure mechanisms of short-glass-fibre reinforced poly(ethylene terephthalate) were investigated with particular attention to the effects of fibre weight fraction (Wf=1 wt%, 30 wt% and 60 wt%). A fracture morphology study was carried out for the surface and for the interior of uniaxial tensile specimens. On the surface, tensile cracks occurring mostly at the fibre ends seemed to be more influential in catastrophic fracture initiation with decreasing Wf. However, the failure mechanisms in the interior were different from those on the surface. For specimens of low Wf (1 wt%), shear bands grew around the fibre ends. A “specific layer” was formed in the matrix along the fibre-matrix interface and shear cracks propagated near the interface in the fibre length direction. The fibre pull-out from the matrix as well as the voiding at the fibre ends, induced by the shear cracks, had strong effect on the fracture initiation. For intermediate and higher Wf (30 wt% and 60 wt%), the shear-band induced cracking near the interface caused matrix shear cracking which was the most influential factor in the fracture initiation. The shear failure in the interior almost dominated the fracture processes throughout the specimens.

Stress fields on and beneath the surface of short-fiber-reinforced composites and their failure mechanisms

Abstract Stress distribution and failure mechanisms around the fiber ends were studied on the surface and in the interior of short-fiber-reinforced composites under uniaxial tensile loading. Microscopical observation of failure mechanisms in short-glass-fiber-reinforced poly(ethylene terephthalate) (SGFR/PET) showed that tensile cracks occurred mostly at the fiber ends on the surface and grew into the matrix. In the interior, however, shear yielding and shear cracking proceeded along the fiber/matrix interface in the fiber length direction with growth of shear bands around the fiber ends. The shear failures occurred preferentially for longer fibers. On the other hand, finite element calculation showed that octahedral shear stresses around the fiber ends in the interior were considerably higher than those on the surface. The shear stresses in the interior became much larger as the fiber aspect ratio increased. This calculation agreed qualitatively with the observation that these shear cracks appeared mostly in the interior.

Effect of plasma treatment of aluminum on the bonding characteristics of aluminum–CFRP composite joints

The effect of surface treatment of aluminum on the bonding characteristics of aluminum- CFRP (carbon fiber reinforced plastic) composite joints has been investigated. The surface of the aluminum panel was treated by DC plasma. The plasma treatment was carried out at different treatment times and volume ratios of acetylene gas to nitrogen gas. The volume ratios used were 1 : 9, 3 : 7, 5 : 5, 7 : 3, and 9 : 1. The treatment times used were 10, 20, 30, 40, 50, and 60 s. Optimal plasma treatment conditions were determined by measuring the T-peel strength and water contact angle as functions of the treatment time and gas volume ratio. Single lap shear tests and T-peel tests were performed using plasma-treated aluminum-CFRP composite joints and regular aluminum-CFRP composite joints to determine the effect of treatment on the shear strength and T-peel strength. The results showed that the water contact angle was minimum and the peel strength was maximized when the aluminum was plasma-treated for 30 s at a volume ratio of 5 : 5. The shear strength of aluminum-CFRP composite joints plasma-treated under the optimal treatment conditions was 33% higher than that of regular aluminum-CFRP composite joints. The T-peel strength of plasma-treated aluminum-CFRP composite joints was almost six times larger than that of regular aluminum-CFRP composite joints. SEM examination showed that the improvement in bond strength was attributed to a uniform spread of the epoxy adhesive due to the surface energy increase of aluminum and this resulted in a cohesive failure of the epoxy adhesive.

Influence of weathering on unreinforced and short glass fibre reinforced thermoplastic polyester

The influence of weathering on fracture toughness, Jc, yield strength, σy, local ultrasonic velocity, VR, and microhardness, Hu, in unreinforced and a short glass-fibre-reinforced polyester-based thermoplastic ″Xenoy″ has been investigated. Unreinforced material weathered for 11 months outdoors in Perth, West Australia, exhibited a significant decrease in VR and Jc, whereas little change was shown in σy, Hu and the fracture surface morphology. Irradiation for 1000 h by artificial ultraviolet rays upon the unreinforced material caused a considerable increase in Hu and only a slight deterioration in VR of the surface layer. Filling with the short fibres induced an improvement in σy and a large reduction in Jc for the unreinforced material. The natural weathering of the reinforced material caused a small reduction in Jc but a large degradation in the slope of the R-curve. It was concluded that the measurement of Jc and the slope of R-curve in combination with VR and Hu was an effective way to study the effects of weathering on engineering plastics.

Fracture Behavior of Unidirectional Commingled-Yarn-Based Carbon Fiber/Polyamide 6 Composite under Three-Point Bending

Flexure-response and fracture behavior of unidirectional commingled-yarn-based carbon fiber (CF)/polyamide (PA) 6 composite were investigated using three-point edgewise and flatwise bend tests. Difference of the experimental flexure data from that expected was attributed to poorly bonded fiber/matrix interfaces and deviated fiber bundle orientations. For the fracture test, four kinds of notch direction were adopted: edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel (ZT) and perpendicular (ZT) to this direction. Strength and absorbed fracture energy for L and ZL specimens exhibited high sensitivity to notching. ZL specimens exhibited the lowest values of the critical stress intensity factor, K c which were slightly superior to those of unfilled PA6 matrix. T specimens showed the largest values of the strain energy release rate Gc which were mostly occupied by the plastic fracture energy: the large plastic energy coincided with generation of the crushing-mode failure on the compression side. Enlargement of the compression area was analyzed by means of the rigidity reduction resulting from the fracture progression.

Toughness and microscopic fracture mechanisms of unfilled and short-glass-fibre-filled poly(cyano arylether)

Fracture mechanisms of an advanced high-strength thermoplastic poly(cyano arylether) (PCAE) and its short-glass-fibre (SGF)-reinforced composites have been studied in relation to toughnesses Kc and Jc. Test temperatures were 23 and 100 °C. Reflected and transmitted optical observations were combined with scanning electron microscopy for the fractographic investigation. For unreinforced PCAE tested at 100 °C, the damage area in front of a notch becomes fairly large in size and consists of numerous tensile microfailures around the local plastic yielding zone, as compared with that tested at 23 °C. This resulted in a substantial improvement of Kc and a big increase in Jc. Filling fibres, however, produced both toughening and anti-toughening results: effects of fibre spanning, pull-out and bridging across the local plastic failure zone and zigzag propagation of fracture due to fibre filling, improved the toughness. However, adhesive failure at the fibre-matrix interface, tensile microcleavage at the fibre ends and straightforward fracture in the skin layer, considerably diminished the values of Kc and Jc, except for the trend of Kc at 23 °C.

Characteristics of Acoustic Emission Wave Attenuation in Short Fiber Reinforced Plastics

The attenuation coefficient (α) of acoustic emission (AE) waves was evalu ated for short glass fiber reinforced poly(ethylene terephthalate) (SGFR-PET) as a func tion of fiber volume fraction (V f ) using simulated AE waves. The coefficient α decreased with increasing Vf more rapidly than expected from a simple linear relation between α and Vf . The effect of wave attenuation was taken into account in a quantitative analysis of the peak amplitude distribution of AE waves which were detected from each zone partitioned in a specimen gage portion. The amplitude distribution which was obtained by the zoning method was compensated for the attenuation loss stated above. The compensated ampli tude distribution was shown to be almost similar in every zone of the specimen. The com pensated amplitudes of AE waves became much larger as the specimen deformation pro ceeded. Consequently, the AE peak amplitude obtained from short fiber reinforced plastics was shown to be considerably influenced by the wave attenuation.

Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP-aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar+ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar+ ion irradiation treatment was carried out at Ar+ ion dose of 1016 ions/cm2. Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.

S-N Curve Deduction of a KTX High-Speed Train Structure for an Accelerated Life Testing

An accelerated fatigue test is essentially required to maintain the reliability of the actual structures of KTX under operation conditions. However, actual fatigue life cannot be obtained because the conventional fatigue tests are not adequate to the real load conditions. Moreover foreign component makers have not provided data of the loading stresses (S) versus cycles at the failure (N). In this study, we suggested a deduction method of the S-N curve for establishing an accelerating test under various load levels. Load history was acquired from the field tests. A Rainflow method was used on the cycle counting of the field load data. After that, an S-N curve was obtained through the iteration process under the condition that the damage index satisfies to 1 in the Miners rule. The deduced S-N curve was applied to the performance evaluation of Korean-made sealed knuckles compared with imports.

Degradation Behavior and Micro-Hardness Analysis of a Coolant Rubber Hose for Automotive Radiator

Rubber hoses for automobile radiators are apt to be degraded and thus failed due to the influence of contacting stresses of air and coolant liquid under thermal and mechanical loadings. The aging behaviors of the skin part of the hoses due to thermo-oxidative and electro-chemical stresses were experimentally analyzed. Through the thermo-oxidative aging test, it was shown that the surface hardness IRHD(International Rubber Hardness Degrees) of the rubber increased with a considerable reduction of failure strain as the aging time and temperature increased. On account of the penetration of coolant liquid into the skin part influenced by the electro-chemical degradation(ECD) test the weight of the rubber hose increased, whereas their failure strain and IRHD hardness decreased. The hardness of the hose in the side of the negative pole was the most deteriorated at the test site of the hose skin just below the coolant surface.